1. Field of the Invention
The present invention relates to semiconductor processing technology and, in particular, concerns a magnetoresistive random access memory (MRAM) device and fabrication process for an element.
2. Description of the Related Art
MRAM is a developing technology that offers the advantages of non-volatility and high density fabrication. MRAM structures employ the spin property of electrons within layers of metallic-based magnetic material that have a physical property known as magnetoresistance (MR) to read the memory storage logic states. Binary logic states typically require sensing of a resistance differential to distinguish between “on” and “off” states. If a particular material has a high resistance, the ability of electrons to flow through the material is inhibited and, conversely, a low resistive material tends to allow a higher degree of current flow. MRAM structures take advantage of this resistivity concept by manipulating the alignment of magnetic fields within metallic layers of material to increase or decrease the resistance.
Moreover, current flow through a conductive trace induces a magnetic field. In the presence of an orthogonal external magnetic field, the spin direction of a stationary MR electron may be altered in one of two directions; either “upspin”, parallel to the magnetic field or “downspin”, antiparallel to the magnetic field. Thus, by the application of a magnetic field, the resistivity of a magnetoresistive material can be altered.
MRAM devices typically consist of a pinned (spin stationary) layer and a sense (spin programmable) layer with a tunneling layer interposed between the pinned layer and the sense layer. Typically, the magnetoresistive structure has electrodes that are positioned adjacent the sense layer and also adjacent the pinned layer such that the application of a current to the pinned and sense layer can change the magnetoresistive qualities of the MRAM element thereby altering its resistance. In this way, logic states can be programmed into an MRAM element and, by subsequent reading of the resistance of the MRAM element via the conductive traces, the logic state can be subsequently retrieved.
MRAM structures are typically formed using known patterning and etching techniques used to manufacture other semiconductor memory devices, such as DRAMs. For example, the typical MRAM element or array is generally formed on a substrate. Sequential layers of magnetic material are deposited on the substrate and are then patterned and etched so as to define the MRAM element. For example, in one common MRAM structure, a magnetic pinned layer is deposited globally over a region of the substrate followed by the global deposition of the tunneling layer and then followed by the global deposition of the sense layer. Patterning and etching or ion milling techniques are then used to selectively remove portions of the globally deposited layers so as to define an MRAM element.
Several difficulties occur during the manufacturing process which reduce the yield of usable MRAM elements. For example, the sense layer and tunneling layer are often patterned prior to the patterning of the underlying pinned layer. During subsequent patterning of the underlying pinned layer, the sidewalls of the patterned sense layer is exposed and material from the underlying pinned layer can sputter up during the pinned layer patterning thereby resulting in the two magnetic layers being shorted together. With the two magnetic layers shorted together, the MRAM element is typically unable to record two separate binary states and be used for memory storage.
This particular problem becomes more exacerbated when MRAM devices of higher density are being made. In particular, higher density devices typically have much smaller features, thereby resulting in the pinned layer and the sense layer being positioned more closely together resulting in shorted MRAM elements being more likely.
A further difficulty with the typical MRAM fabrication process is that once the sense layer and tunneling layer have been patterned, it is often very difficult to pattern the underlying pinned layer to have dimensions other than the previously patterned sense layer. Consequently, in the typical MRAM element, the outer lateral edges of the patterned sense layer and the underlying patterned pinned layer are generally located proximate to each other. As is understood, the magnetic field of the underlying pinned layer is substantially stronger than the magnetic field of the sense layer. Magnetic flux emanating from the pinned layer can alter or otherwise effect the magnetic field of the sense layer. Hence, the magnetic field of the pinned layer can result in variations in the magnetic field of the sense layer other than what has been programmed. Consequently, the data that is stored in the MRAM element can then be inadvertently altered or changed as a result of magnetic coupling.
It is also understood that magnetic flux is more concentrated at the lateral edges of the magnetic pinned layer. Consequently, when the lateral edges of the pinned layer are location proximate the lateral edges of the sense layer, the potential of the pinned layer adversely affecting the sense layer is increased. As MRAM elements are being made increasingly smaller, the proximity of the edges to each other results in an increase in magnetic coupling between the pinned layer and the sense layer.
From the foregoing, it will be appreciated that there is a need for a process of fabricating MRAM elements such that the yield of the MRAM element can be improved and also such that magnetic coupling between the various layers of the MRAM element can be reduced.